1. Field of the Invention
The present invention relates to a display unit which color-reduces multi-tone display data by means of a systematic dither method or an error diffusion method for storage in a frame memory and carries out display and a method therefor.
2. Description of the Prior Art
A prior-art method color-reduces multi-tone display data using a systematic dither method or an error diffusion method for displaying the display data on a display unit such as an LCD or the like having a small number of tone-representing bits.
Originally, this method was employed because multi-tone representation was difficult in terms of the performance of a display device itself such as an LCD or the like.
However, in recent years, through improvements in tone-representing performance of the device itself, rather than in reductions in display memory, there are a growing number of cases where the unit electric power consumption and costs are reduced by design.
Japanese Unexamined Patent Publication No. Hei-9-50262 discloses a technique in which the systematic dither method is employed.
Japanese Unexamined Patent Publication No. Hei-6-138858 discloses a technique in which the error diffusion method is employed.
In these publications, multi-bit display data (for example, each RGB component has 8 bits, 6 bits or the like) is color-reduced to 12 bits (4,096 colors). Hereinafter, for convenience of explanation, an example where multi-bit data is color-reduced to 12 bits (4,096 colors) is cited. However, the present invention is also applicable to a case where another color reduction is carried out, as long as it does not deviate from the spirit of the invention.
In such publications, when performing the color reduction to 12 bits (4,096 colors), 4 bits are distributed to each RGB component, respectively. In addition, there are examples of an 8-bit color system provided as R:G:B=3:3:2 (bits) and a 16-bit color system provided as R:G:B=5:6:5 (bits). These ratios are based on the idea that it is satisfactory that the bit numbers are roughly uniformly allotted among the RGB. When the bit number is not completely uniformly assigned, 1 bit is merely reduced (8 bits) or increased (16 bits) at the most.
However, such distribution deviates from human visual performance, detailed reasons of which will be described later. As a result, the display quality is poor due to the G component being too small Also, the amount of information is wasted due to the B component being too large.
In more detail, since the G component is too small, a sense of unevenness between adjacent pixels, a pseudo-outline may result. Since the B component is too large, the requisite amount of memory unnecessarily increases, causing wasteful electric power consumption and rising costs. This drawback is the first problem.
Now, the second problem, which relates to a second object of the present invention, will be described referring to FIG. 8 through FIG. 11. FIG. 8 is a block diagram of a prior display unit.
In FIG. 8, a pseudo-tone processing means 1 receives inputs of display data (in the present example, each RGB has 6 bits, however, each RGB may have 8 bits) and color-reduces the data by pseudo-tone processing so that each RGB has 4 bits and the total becomes 12 bits (4,096 colors). Herein, the pseudo-tone processing by the pseudo-tone processing means 1 may be either a systematic dither method or an error diffusion method.
A frame memory 2 stores the data after color deduction to be output from the pseudo-tone processing means 1. Herein, since each RGB is color-reduced to 4 bits, the frame memory 2 has a capacity to store 12 bits per one pixel.
A drive means 3 drives an LCD 4 based on the data of the frame memory 2. Herein, the LCD 4 is used as a display device, however, a CRT or a plasma display may be used.
And, in the prior art, display has been carried out with each RGB of 4 bits based on the color-reduced data, which is stored in the frame memory 2.
In recent years, owing to the advancement of technology, an LCD can display 6 bits (64 tones) in some cases. Illustration of reflectance characteristics of the LCD which can display 64 tones is as shown in FIG. 9.
In addition, illustration of reflectance characteristics of an LCD with 4 bits (16 tones) after color reduction is as shown in FIG. 10.
When driving an LCD in a practical manner, the more visually uniform the intervals of the tone data, the more smoothly the tones change, and unclear colors and the like can be prevented.
Therefore, in order to cancel out the reflectance characteristics of FIG. 1, carrying out a correction using the characteristics of FIG. 11 by means of the drive means 3 of FIG. 8 can be considered.
However, in the prior art, as can be clearly understood referring to FIG. 11, tones that can be displayed become dispersive even after this correction. This point is particularly remarkable in halftones where irregular colors easily become conspicuous, thus the display quality of appearance has been unsatisfactory. This drawback is the second problem.